This invention relates to an exhaust emission control apparatus for a combustion engine, including a hydrocarbon adsorbent for adsorbing hydrocarbons at a lower temperature, and a catalytic converter being activated at a higher temperature.
In Japanese Patent Application Laid-open No.10178/93, a hydrocarbon adsorbent and a three-way catalytic converter are disposed in series from an upstream side in an exhaust pipe connected to a combustion engine. This hydrocarbon adsorbent adsorbs hydrocarbons in the exhaust gas at a lower temperature such as the start-up period of the engine, and desorbs it at a higher temperature. The three-way catalytic converter disposed downstream of the hydrocarbon adsorbent purifies hydrocarbons desorbed from the hydrocarbon adsorbent.
During the start-up period of the engine at a lower temperature, fuel is not effectively mixed with intake air. Therefore, to warm up the engine, the air fuel mixture is controlled to be rich relative to stoichiometric.
Then the temperature of the engine rises, the hydrocarbon adsorbent begins desorbing hydrocarbons. At this time, the air fuel mixture is controlled to be lean relative to stoichiometric so that the air fuel mixture entering the catalytic converter does not get too rich. Therefore, even if the hydrocarbon adsorbent desorbs hydrocarbons, the three-way catalytic converter performs oxidation of hydrocarbons (HC) and carbon monoxide (CO), and reduction of nitrogen oxides (NOx).
After desorbing hydrocarbons from the hydrocarbon adsorbent, the air fuel mixture is controlled to be stoichiometric.
According to this disclosure, during the start-up period of the engine at a lower temperature, the engine may maintain steady state by controlling the air fuel mixture to be rich.
However, as the inventors of the present invention have discovered, even if the air fuel mixture is controlled to be rich, a sufficient rise of the exhaust gas temperature is not obtained, and activation of the three-way catalytic converter is not expected. Therefore, it is necessary to install a second air supply device separately so that the exhaust temperature may be raised. This increases cost.
On the other hand, when the hydrocarbon adsorbent desorbs hydrocarbon, the air fuel mixture is controlled to be lean relative to stoichiometric. Therefore, the rapid rise of the exhaust gas temperature is not obtained in this period, and early activation of the three-way catalytic converter is not expected either.
Generally, the desorbing temperature of the hydrocarbon adsorbent is lower than the activation temperature of the three-way catalytic converter.
Therefore, the period after reaching the desorbing temperature of the hydrocarbon adsorbent until reaching the activation temperature of the three-way catalytic converter becomes a matter of concern. In this period, the three-way catalytic converter might not purify the hydrocarbon desorbed from the hydrocarbon adsorbent. In other words, the amount of hydrocarbons discharged to atmosphere might increase until the three-way catalytic converter activates.